1. Field of the Invention
The present invention relates to a process for producing 3-methyltetrahydrofuran, and processes for producing 3-hydroxy-3-methyltetrahydrofuran and 3-methyldihydrofuran, which are intermediates thereof. 3-Methyltetrahydrofuran obtained by the present invention is useful as a starting material of polyether polyol, which is, for example, a component of a thermoplastic polyurethane, or a solvent. 3-Hydroxy-3-methyltetrahydrofuran and 3-methyldihydrofuran obtained by the present invention are useful as starting materials for chemicals such as medicines or agricultural chemicals.
2. Description of the Background
The following conventional processes for producing 3-methyltetrahydrofuran are known: (a) a process of cyclodehydrating 2-methyl-1,4-butanediol [see Ind. Eng. Chem. Res., 33, pp. 444-447 (1994)], (b) a process of hydrogenating methylsuccinic acid, using isopropanol as a hydrogen source, over hydrous zirconium oxide catalyst [see Bull. Chem. Soc. Jpn., 65, pp. 262-266 (1992), (c) a process of hydroformylating methallyl alcohol, hydrogenating the resultant formylated product, and then cyclodehydrating the hydrogenated product [sec J. Prakt. Chem., 314, pp. 840-850 (1972)], and (d) a process of hydrogenating 3-methyl-3,4-epoxybutan-1-ol in an acidic aqueous solution [see U.S. Pat. No. 3,956,318].
However, in process (a), it is difficult to obtain 2-methyl-1,4-butanediol industrially, which is a starting material. In process (b), preparing hydrous zirconium oxide catalyst is complicated, and acetone is formed as by-product in an equivalent to the amount of isopropanol used as the hydrogen source. In process (c), a rhodium compound used as a catalyst for the hydroformylation reaction is expensive. Moreover, methallyl alcohol, which is a starting material, is not industrially produced so that it is difficult to obtain the alcohol easily and economically. In process (d), 3-methyl-3,4-epoxybutan-1-ol is not industrially produced so that it is difficult to obtain the compound easily and economically. Besides, under reaction conditions in the acidic aqueous solution, the starting material tends to be hydrolyzed to produce a triol as by-product, which is formed by ring-opening of the epoxy ring. Therefore, this is not an industrially favorable processes for producing 3-methyltetrahydrofuran.
As processes for producing 3-hydroxy-3-methyltetrahydrofuran, the following processes are known: (e) a process of reacting 2-hydroxyethyl-2-methyloxirane with tributyltin methoxide and then thermally decomposing the resultant tin compound to obtain the target compound [see Tetrahedron, 38, pp. 2139-2146 (1982)], (f) a process of decomposing .beta.-tetrahydrofurylmethanol on an alumina catalyst [see Bull. Soc. Chim. Fr., 5-6, Pt 2, pp. 261-266 (1980)], (g) a process of reacting 4-chloro-3-methyl-1,3-butanediol with KCN [see Nippon Kagaku Kaishi, pp. 1021-1025 (1977)], (h) a process of decomposing 4,4-dimethyl-1,3-dioxane [see Meiji Daigaku Kagaku Gijutu Kenkyusho Nempo, 17, p. 22 (1975)].
However, all of the above-mentioned processes have problems. In process (e), an expensive compound must be used to produce 3-hydroxy-3-methyltetrahydrofuran, and selectivity of 3-hydroxy-3-methyltetrahydrofuran is as low as 50%. In process (f), the raw material cannot be easily available, and a high temperature of 300.degree. C., or higher is necessary for the decomposition. In process (g), the main reaction thereof is cyanization and thus 3-hydroxy-3-methyltetrahydrofuran can be obtained only as a by-product. In process (h), 3-hydroxy-3-methyltetrahydrofuran can be obtained in only a little amount as a by-product, and cannot be selectively obtained.
For such reasons, any industrially-established process for producing 3-hydroxy-3-methyltetrahydrofuran has not been known until now.
As processes for producing 3-methyldihydrofuran, in particular, 3-methyl-4,5-dihydrofuran or 3-methyl-2,5-dihydrofuran, the following processes are known: (i) a process of hydroformylating methacrolein diethylacetal, reducing its aldehyde group, cyclizing the resultant alcohol compound, and subjecting eliminating ethanol from the cyclic compound to obtain 3-methyl-4,5-dihydrofuran [see J. Org. Chem., 37, p. 1835 (1972)], (j) a process of isomerizing 3-methyl-3,4-epoxy-1-butene in the presence of a higher tertiary amine and a zinc oxide catalyst to obtain 3-methyl-2,5-dihydrofuran [see WO 91/13882], (k) a process of isomerizing isoprene oxide in the presence of iron acetylacetonate (Fe(acac).sub.3) and hydrogen iodide to obtain 3-methyl-2,5-dihydrofuran [see U.S. Pat. No. 3,932,468], (l) a process of reacting hydroxyacetone with vinyltriphenylphosphonium bromide, and following to cyclization reaction of the resultant to obtain 3-methyl-2,5-dihydrofuran [see J. Org. Chem., 33, p. 583 (1968)].
However, in process (i), methacrolein, which is not stable, must be converted to the acetal, a rhodium catalyst using in the hydroformylation reaction is very expensive, and it needs the number of steps. In process (j), the preparation of the catalyst is difficult. In process (k), it is necessary to use hydrogen iodide which is highly corrosive. In process (l), a large amount of expensive triphenylphosphine is necessary to prepare vinyltriphenylphosphonium bromide, which is used in a mole equivalent to hydroxyacetone. Therefore, these processes are not industrially-profitable processes for producing 3-methyldihydrofuran.